This one is a bit more tricky than Anubis other achievement. You need to kill an opponent with your ultimate/4 through a wall. Your ultimate is a long laser that does a pretty good amount of damage. You manually control it and can not move from the position after you start using it.
Any part of the map that your character can not move through is a wall, this includes edges of the map or corners of things inside the map even if you can actually see through they still count as walls.
If opponents are in lane and insight of your minions/tower/allies then you can see them through the map.
Arena has the pillars in the middle and the little buff camps on the sides, Clash has pillars in the middle plus the jungle areas on each side of the map, Conquest has jungle areas and the broken parts of the Fire Giant Temple, Siege has a jungle, Assault has its zig-zag corners and walls behind the Phoenix and Joust has its two jungles.
Your best bet is to chill in a jungle and wait for a opponent to get low on health and then hit them with your ultimate, you might even want to lead off with your 3 ability Grasping Hands witch will slow them down and deal some damage which both can help.
I'm not sure if the towers/Phoenix spawns count as walls but worse case you can try getting kills from behind those two too.